1. Field of the Invention
The present invention provides a droplet ejection device for a highly viscous liquid, particularly an ejection device capable of intermittent discharge of highly viscous liquid droplets.
2. Description of the Related Art
Digital ejection technology usually atomizes liquid for ejection purposes. However, the application of this technology is constrained by the viscosity requirement as low as a few tens cP. Digital ejection technology was first applied to packing machines in the early years, serving to further identify the features and conditions of the product by ejecting droplets to the target subject to form appropriate images and characters. Such packing machines are available through Japanese companies such as Marsh (U.S. Pat. No. 4,378,564) and Hitachi (U.S. Pat. No. 4,849,909). However, as mentioned earlier, digital ejection technology works only with liquid whose viscosity is no more than a few tens cP, and whose droplet diameter is approximately 0.5 μm; equivalent to several tens pL in volume. On the other hand, paste-like highly viscous liquid with more than thousands or even tens of thousands cP in viscosity is commonly applied to ejection devices in glue spreaders, sprayers, or food machines, etc. Given its extremely high viscosity, such kind of liquid can only be ejected in large quantities by the machinery equipment or applied to the surface of the target subject in large areas, unable to be atomized for digital ejection. While highly viscous liquid, such as adhesives or tin paste, can be melted out by heating to reduce its viscosity to meet the requirement of digital ejection, it is also certain that, by doing so, the physical and chemical properties of the fluid will be changed, too.
FIG. 10 illustrates both the ejecting amount and the viscosity of an ejected droplet, wherein the X-axis represents ejecting amount, or unit volume of every intermittent ejection, while the Y-axis represents liquid viscosity. By referring to the figure, the performance and the limitations of current liquid ejection devices can be easily understood. Take an ink jet printer, for instance. The viscosity of ink jet ranges from 1 cP to 10 cP, whereas the ejecting amount of the intermittently jetted ink reaches from 1 pL to 100 pL. Thanks to its low viscosity, the ink jet can be atomized to the advantage of image configuration. On the other hand, for liquid with higher viscosity, such as spray adhesives whose viscosity amounts to approximately 10,000 cP, the ejecting amount of every intermittent ejection can be more than 10,000 pL. Accordingly, every single ejection of the spray adhesives should be considered a continuous stream unable to be atomized.
In view of the advancement of micro-electro-mechanical technologies in recent years, new technologies that are able to directly issue paste-like liquid and to precisely control the size of the liquid droplet at the same time will play a significant role in promoting direct ejection for microcircuit printing technology. Therefore, it is desirable to develop a droplet ejection device for a highly viscous liquid to control the size of the droplet while ejecting highly viscous liquid.